The typical leadoff hitter can usually run. He has a high on base percentage, good average and takes his walks. The leadoff hitter can handle the bat by bunting, good hit and run guy and doesn’t strike out a lot. He can create havoc on the bases when necessary. -- ProBaseballInsider.com
That seems a good general overview of the requirements. I'd add an ability to work the count. Especially first and second time through the order, you don't want your lead-off man grounding out on the first pitch, but giving those behind him as many chances as possible to see what the opposing pitcher is throwing. Segura certainly thinks he's a good lead-off guy, saying last month, "I always love being at the top of the lineup because I think that’s my game. I can run, steal some bases, and I’m a contact guy. I’ll hit ground balls and get you line drives. When you make contact and hit a lot of balls, it’s good to have a guy like that at the top of the lineup."
But let's take a look at the various components and see how true that is.
High on-base percentage
On the surface, Segura checks out. With an on-base percentage of .346, Segura is well above league average OBP of .319. But that is depressed by the pitchers' hitting. .346 is actually fractionally below the mean (.347) for National League leadoff hitters this year. As we'll see, Segura's figure also very heavily driven by his batting average, and while he's hitting over .300, that is helped by a BABIP of .344. That is not the result of a high line-drive rate (19%; career average is 18%), so much as his dozen infield hits. That's always been a significant part of his arsenal: over 2013-15, 6.6% of his PA in Milwaukee ended in infield hits, though that figure is down to 4.2% this year.
Can handle the bat by bunting
But guess how many bunt hits Segura has this year? The correct answer is: one, against the Blue Jays yesterday. His last was all the way back on August 20, 2014. He seems to have only made one other attempt this year, that was not a sacrifice (the others were with no outs and a runner on second): leading off the seventh inning on April 16 in San Diego, when we were down 3-2. The catcher threw him out. It's not often I call for more bunting, but dammit... Segura has been in the top 20 for infield hit percentage since the start of 2013. Not bunting, at least occasionally, seems like he's leaving a significant number of potential hits out there on the table.
Takes his walks
No: Segura doesn't. Indeed, he makes Chris Owings - and not the recent version, but the hacktastic creature we saw in 2014-15 - look like the model of plate discipline. Over those two seasons, Owings had a BB% of 4.8%. This year, through the start of play today, Segura had managed 14 walks in 310 plate-appearances, or 4.5%. There's only 10 qualified hitters with a lower walk-rate in the National League (though you might get a kick of who has the lowest!). Now, that's nothing new: indeed, since 2013, Segura's BB% is league worst. It has just been the way he is. But the team had no problems Segura changing his stance - was asking for better pitch selection too much to ask?
That said, Jean is actually swinging at pitches out of the zone at a lower rate than previously - 30.7%, down from 38.6% in 2015, though that is still more than average. However, this has been accompanied by an even bigger reduction for Segura's swing-rate on pitches in the zone, which has dropped from 68.1% to 60.4. So, he's taking more balls, but also more called strikes: He has increased his overall pitch-count per at-bat somewhat from 3.52 to 3.78, but that's still lower than the average for major-league hitters. At the risk of stating the obvious, the longer your at-bats, the more often you'll walk; Segura still is barely even mediocre in the former, and flat-out awful at the latter.
Doesn't strike out a lot
This is an area where Segura has been good: his career and season K-rates are both below 15%, compared to an MLB average of 20%. Add the low walk-rate, and about four in five Segura PAs will end with the ball being put into play; we all know how much D-backs assistant hitting coach Mark Grace loves him some balls in play. Of course, you can argue - and, indeed, I've done so quite a bit - that strikeouts are generally no worse than other outs, because any value offered by the "productive out" is countered by the lethal effect of the double-play. However, balls in play are good for leadoff hitters, because they tend to be speedier types who can turn more of those into hits.
Can create havoc on the bases
Segura certainly creates havoc. But an equal opportunity mayhem, which destroys virtually as much as it creates. He leads the team in stolen-bases... and caught stealing. He takes the extra base more often than anyone else... and has as many OOB (outs on basepaths, not counting CS and PO) as any other two D=backs combined. All told, his base-running value is... okay. Which is actually better than I expected; selective memory tends to remember the time he runs into outs, while forgetting the occasions he goes from first to third on a single, and no-one really notices. Good base-running is like good umpiring; if it's noticed, you're probably doing something wrong. .
Who are the alternatives
Make no mistake. I like Segura, not least because his continued success helps prove a certain ESPN pundit wrong again. He has made the transition to being an everyday second baseman almost seamlessly, and his rebound after a horrific personal tragedy, is one of the best stories on the 2016 Diamondbacks. But none of those things make him a good fit in the lead-off spot for Arizona, and his skills would be better off down the order. The problem is, does the team have anyone who would be a better fit?
For a good lead-off hitter tends to be a good hitter generally. If we had two Paul Goldschmidt's, one would be the greatest lead-off hitter in franchise history. The same goes with other high OBP players, such as Jake Lamb, yet their power makes them a better fit lower in the order. You really want a high OBP player who hits a lots of singles, and steals bases. Chris Owings might be a good choice: his OBP of .340 is almost the same as Segura, although at .353, is even more BABIP dependent. But he's arguably a better base-runner, being 8:0 in SB attempts. Of course, the real answer is A.J. Pollock, rehabbing a broken elbow, and he may not return this year.
I started off this piece without a question in the title: it was "Why Segura shouldn't bat lead-off," frustrated after watching yet another TOOTBLAN by him. But on further analysis. it seems he's not as bad as I feared. In particular, while he may be imperfect, there doesn't seem to be anyone else on the current roster who could really be considered as a better alternative for the spot. As long as Segura's general level of performance stays at what he has managed since his arrival, not the much lower standard of his previous season in Milwaukee, he'll be fine. Just consider who might be hitting there right now, if we hadn't dealt for Jean this winter.